Digital speech communication systems including voice storage and voice response facilities utilize signal compression to reduce the bit rate needed for storage and/or transmission. One well-known digital speech coding system, such as disclosed in U.S. Pat. No. 3,624,302, issued Nov. 30, 1971, includes linear prediction analysis of an input speech signal. The speech signal is partioned into successive intervals and a set of parameters representative of an interval of speech is generated. The parameter set includes linear prediction coefficient signals representative of the spectral envelope of the speech in the interval, and the pitch and voicing signal corresponding to the speech excitation. These parameter signals may be encoded at a much lower bit rate than the speech signal wave form itself. A replica of the input speech signal is formed from the parameter signal codes by synthesis. The synthesizer arrangement generally comprises a model of the vocal tract in which the excitation pulses are modified by the spectral envelope representative prediction coefficients in an all pole predictive filter. Whereas this type of pitch excited linear predictive coding is very efficient, the produced speech replica exhibits a synthetic quality that is often difficult to understand.
Another known digital speech coding system is disclosed in U.S. Pat. No. 4,472,832, issued Sept. 18, 1984. In this analysis and synthesis system, LPC parameters and a modified residual signal for excitation are transmitted. The excitation signal is a sequence of pulses selected from the peaks of the cross-correlation of the LPC filter impulse response and the original signal. This type of excitation is often referred to in the art as multi-pulse excitation. Whereas this system produces a good speech replica, it is limited to minimum bit rates of approximately 9.6 kilobits per second (Kbs). In addition, during the voiced regions, the speech replica tends to have a detectable roughness. Also, the method requires a large number of complex calculations.
In view of the foregoing, there exists a need for an analysis and synthesis system that is capable of producing an accurate speech replica during the voiced period of a speech wave and also during the unvoiced regions of the speech wave. In addition, it is desirable to have a lower bit rate.